This invention relates to apparatus for determining the rate of flow of a fluid by sensing the force exerted by the fluid on a moveable element in the flow stream.
Target meters are commercially available to measure the flow rates of fluids. They are generally relatively simple, low cost devices consisting basically of a target, such as a disc, suspended in the flow stream by a rod that is attached to a force transducer. The force or torque arising from flowing fluid impacting the disc is sensed by the transducer, which provides an electrical output signal responsive to the flow rate of the fluid. These meters provide typical accuracies specified as several percent of span over a 10 to 1 flow range. The relatively poor accuracy of these meters at the lower flow rates limits their applications, and it is an object of this invention to improve their accuracy.
The above and other objects are attained by target meters in accordance with various preferred embodiments of the present invention. Preferred embodiments of the present invention use a target in a flowing fluid where the orientation of the target to the direction of flow of the fluid can be selectively changed between two or more orientations, and where the target provides a different flow impedance in each of the orientations. This change in flow impedance gives rise to a corresponding difference in drag forces exerted on the target by the flowing fluid. Those forces, or the displacement associated with them are measured to determine the rate of fluid flow.
In some embodiments, the target orientation is changed by continuous rotation and electrical signals responsive to the variable drag on that target are processed to provide a measurement of fluid flow rate. In one preferred embodiment, the target is in the form of a rigid vane, extending radially outwards from a shaft which is attached to a transducer and rotated in the fluid flow stream. The drag forces imposed on the vane by the stream cyclically vary from a maximum to a minimum and are converted by the transducer into corresponding electrical signals. The magnitude of only the variational component of the electrical signal is used to determine flow rate. The transducer signals, which may form part of its output signal as may be necessitated by its power requirements, either AC or DC, or friction related loads, for example, are not used. By this means, the flow sensing error and in particular transducer zero drift relating to the conversion of target drag to a flow rate signal at low flow rates, is reduced thereby enabling the meter to be effectively used at lower flow rates.
In another preferred embodiment the angular orientation of a vane in the fluid flow stream is cyclically oscillated in alternate directions whereby the corresponding variation in drag forces are used by the transducer to provide flow rate responsive signals as in the first embodiment.
Although it is believed that the foregoing recital of features and advantages may be of use to one who is skilled in the art and wishes to learn how to practice the invention, it will be recognized that the foregoing is not intended to list all of the features and advantages. Moreover, it may be noted that various embodiments of the invention may provide various combinations of the hereinbefore recited features and advantages of the invention, and that less than all of the recited features and advantages or the invention, my be provided by some embodiments.